Bulk polymerization process

ABSTRACT

A method for the continuous production of polydienes, the method comprising the steps of (a) charging a mixture of one or more monomer, catalyst system, and less than 50% weight percent organic solvent based on the total weight of the monomer, catalyst and solvent, into first vessel, (b) polymerizing the monomer to a conversion of up to 20% by weight of the monomer to form a mixture of reactive polymer and monomer, (c) removing the mixture of reactive polymer and monomer from the vessel, and (d) terminating the reactive polymer prior to a total monomer conversion of 25% by weight.

This application is a continuation of U.S. patent application Ser. No.11/062,673, now U.S. Pat. No. 7,351,776, filed Feb. 22, 2005, whichclaims the benefit of U.S. Provisional Application No. 60/549,521, filedon Mar. 2, 2004, which are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to process for the bulk polymerization ofmonomer.

BACKGROUND OF THE INVENTION

In bulk polymerization (also called mass polymerization), the reactionmedium is typically solventless; i.e., the monomer may be polymerized inthe absence or substantial absence of any solvent, and in effect, themonomer itself may act as a diluent. Since bulk polymerization involvesmainly monomer and catalyst, there is reduced potential forcontamination and the product separation can be simplified. Economicadvantages include lower capital cost for new plant capacity, lowerenergy cost to operate, and fewer people to operate may be realized. Thesolventless feature can also provide environmental advantages withreduced emissions and wastewater pollution.

Nonetheless, bulk polymerization can require careful temperaturecontrol, and there may be a need for strong and elaborate stirringequipment since the viscosity of the polymerization system can becomevery high. In the absence of added diluent, the cement viscosity andexotherm effects can make temperature control very difficult. Also,cis-1,4-polybutadiene is insoluble in 1,3-butadiene monomer at elevatedtemperatures.

For these reasons, bulk polymerization processes have not proven to becommercially successful. Since the advantages associated with bulksystems are very attractive, there is a need to improve thepolymerization systems that are conducted in bulk.

SUMMARY OF THE INVENTION

The present invention also provides a method for the continuousproduction of polydienes, the method comprising the steps of (a)charging a mixture of one or more monomer, catalyst system, and lessthan 50% weight percent organic solvent based on the total weight of themonomer, catalyst and solvent, into first vessel, (b) polymerizing themonomer to a conversion of up to 20% by weight of the monomer to form amixture of reactive polymer and monomer, (c) removing the mixture ofreactive polymer and monomer from the vessel, and (d) terminating thereactive polymer prior to a total monomer conversion of 25% by weight.

The present invention also provides a polymer of diene monomers. Thepolymer when derived from a lanthanide-based catalyst has a cis contentof at least 97, and has at least one end that is functionalized when itis derived from a precursor with at least 40% live ends. The polymer mayalso be derived from a neodymium-based catalyst system, and have aMooney viscosity (ML₁₊₄ at 100° C.) of from about 15 to about 45. Thepolymer may also be derived from a cobalt-based catalyst system, and bea branched polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatical depiction of an apparatus configurationuseful for practicing the subject invention.

FIG. 2 is a diagrammatical depiction of a vessel for practicing thefirst stage of the process of this invention by employing a cobaltcatalyst system.

FIG. 3 is a diagrammatical depiction of a vessel for practicing thefirst stage of the process of this invention by employing a lanthanidecatalyst system.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention provides a method for the continuous production ofpolydienes, the method comprising the steps of (a) adding liquidconjugated diene monomer and a coordination catalyst system to a firstvessel to achieve a liquid polymerization medium, where thepolymerization medium includes less than 50% by weight organic solvent,(b) allowing the catalyst to polymerize the monomer into polydienes,which increases the temperature of the polymerization medium andconverts a portion of the monomer to a gas-phase monomer, (c) agitatingthe polymerization medium to thereby facilitate conversion of a portionof the monomer to the gas-phase monomer, (d) removing the gas-phasemonomer from the vessel, (e) optionally condensing the gas-phase monomerthat is removed from the vessel to liquid monomer, (f) optionallytransferring the condensed liquid monomer back to the first vessel, (g)removing a portion of the polymerization medium from the first vessel inorder to maintain a monomer concentration within the first vessel of atleast 80% by weight of the polymerization medium within the firstvessel, and in order to maintain a head space within the first vesselthat is about 40 to about 60% of the volume of the first vessel, wheresaid portion of the polymerization medium that is removed includesresidual monomer, (h) transferring the polymerization medium removedfrom the first vessel to a second vessel, (i) agitating and maintainingthe flow of the polymerization medium within the second vessel, (j)maintaining sufficient temperature within the second vessel in order toconvert a portion of the residual monomer to a gas-phase residualmonomer, (k) optionally condensing the gas-phase residual monomerremoved from the second reactor to liquid monomer, (l) optionallytransferring liquid monomer converted from the gas-phase monomer removedfrom the second vessel to the first vessel, (m) optionally adding afunctionalizing agent to the second vessel in order functionalizepolydienes within the second reactor, (n) adding a quenching agent tothe second vessel in order to terminate the further polymerization ofmonomer, and (o) recovering polydienes.

In one or more embodiments, the multi-stage bulk polymerization processof this invention includes a first stage wherein partial polymerizationof available monomer is achieved in the bulk phase followed by a secondstage where at least a portion of unreacted monomer is removed and thedegree of polymerization is controlled.

Within the first stage, monomer is allowed to polymerize to a maximumconversion of up to about 20%, which refers to a polymerization of up toabout 20% of the available monomer. In one embodiment, the maximumconversion is up to about 15%, in other embodiments up to about 12%, andin other embodiments up to about 10%.

Polymerization is conducted within a bulk system, which generally refersto the fact that the system includes less than 50%, in other embodimentsless than 20%, in other embodiments less than about 10%, in otherembodiments less than about 5%, and in other embodiments less than about2% by weight organic solvent based on the total weight of the monomer,polymer, and solvent within the system. In one embodiment, the processis carried out in the substantial absence of an organic solvent ordiluent, which refers to the absence of that amount of solvent thatwould otherwise have an appreciable impact on the polymerizationprocess. Stated another way, those skilled in the art will appreciatethe benefits of bulk polymerization processes (i.e., processes wheremonomer acts as the solvent), and therefore the process of thisinvention may be conducted in the presence of less organic solvent thanwill deleteriously impact the benefits sought by conducting the processin bulk. In another embodiment, the process may be carried out in theabsence of an organic solvent or diluent other than those organicsolvents or diluents that are inherent to the raw materials employed. Inyet another embodiment, the polymerization system is devoid of organicsolvent.

The term organic solvent or diluent is used herein conventionally; thatis, it refers to organic compounds that will not polymerize or enterinto the structure of the polymer to be produced. Typically, theseorganic solvents are non-reactive or inert to the catalyst composition.Exemplary organic solvents include aromatic hydrocarbons, aliphatichydrocarbons, and cycloaliphatic hydrocarbons. Non-limiting examples ofaromatic hydrocarbons include benzene, toluene, xylenes, ethylbenzene,diethylbenzene, and mesitylene. Non-limiting examples of aliphatichydrocarbons include n-pentane, n-hexane, n-heptane, n-octane, n-nonane,n-decane, isopentane, isohexanes, isopentanes, isooctanes,2,2-dimethylbutane, petroleum ether, kerosene, and petroleum spirits.And, non-limiting examples of cycloaliphatic hydrocarbons includecyclopentane, cyclohexane, methylcyclopentane, and methylcyclohexane.Commercial mixtures of the above hydrocarbons may also be used.

Other examples of organic solvents include high-boiling hydrocarbons ofhigh molecular weights, such as paraffinic oil, aromatic oil, or otherhydrocarbon oils that are commonly used to oil-extend polymers. Sincethese hydrocarbons are non-volatile, they typically do not requireseparation and remain with the polymer. The performance characteristicsof the polymer are generally not affected appreciably when the contentof high molecular weight hydrocarbons is less than about 5% by weight ofthe polymer.

The first step is typically initiated by charging monomer and catalystsystem to a reaction vessel. Because the polymerization can be carriedout as a batch process, a continuous process, or a semi-continuousprocess, the manner in which the monomer and catalyst system are chargedmay vary. Also, the manner in which the monomer and catalyst system arecharged may vary based upon the catalyst system employed.

In one or more embodiments, the first stage of the process includes acontinuous polymerization process whereby catalyst and monomer arecontinuously fed to a vessel and a portion of the polymerization mediumis continuously removed from the vessel. Inasmuch as the degree ofpolymerization or monomer conversion is controlled in the first stage,the polymerization medium removed from the vessel may include monomer,polymer, and residual catalyst.

The monomers that can be polymerized according to the process of thisinvention include volatile monomers and optionally non-volatilemonomers. Volatile monomers include monomers that are sufficientlyvolatile to allow heat removal by vaporization of unreacted monomer at arate equal to the rate at which heat is generated by the polymerizationreaction and at a temperature that will allow the formation of thedesired polymer product. In one or more embodiments, the volatilemonomers include those monomers that have a boiling point that is atleast 10° C., in other embodiments at least 20° C., and in otherembodiments at least 30° C. lower than the desired polymerizationtemperature. In a particular embodiment, the monomers are devoid ofhalogenation; for example, vinyl chloride monomers are excluded.

Exemplary volatile monomers include, without limitation, conjugateddienes such as 1,3-butadiene, isoprene, 1,3-pentadiene, 1,3-hexadiene,2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene,2-methyl-1,3-penta-diene, 3-methyl-1,3-pentadiene,4-methyl-1,3-pentadiene, and 2,4-hexadiene. Useful α-olefins includeethylene, propylene, 1-butene, and 1-pentene.

In certain embodiments, it may be beneficial to control the humidity(i.e., water content) of the monomer. For example, where certainlanthanide-based catalyst systems are employed, it may be beneficial todry the monomer. In one embodiment, where a lanthanide-based catalystsystem is employed, the level of water within the monomer is reducedbelow about 20 ppm, in other embodiments below about 10 ppm, in otherembodiments below about 5 ppm, and in other embodiments below about 3ppm.

On the other hand, where certain cobalt-based catalyst systems areemployed, it may prove useful to employ monomer having a higher humiditylevel. In one embodiment, where a cobalt-based catalyst system isemployed, the monomer preferably has from about 30 to about 50 ppm, morepreferably from about 35 to about 45 ppm, and even more preferably fromabout 38 to about 42 ppm water.

As those skilled in the art will appreciate, the level of water withinthe monomer can be adjusted. For example, where the monomer is ratherwet (i.e., contains some level of water) the monomer can be dried usingconventional techniques to achieve that desired water level. On theother hand, where the monomer employed is relatively dry, humidificationof the monomer can be achieved. There are several methods known to thoseskilled in the art for humidifying the monomer. An exemplary systememploys a humidification column including a bed of packing material, alayer of water, and a head space above the water. An inlet adjacent tothe lower end of the cavity receives a dry hydrocarbon stream which isbroken up by the packing material and dissolves water as it passestherethrough. Entrained water droplets fall out of the wet bed in adisengagement zone above the water layer leaving the hydrocarbon streamhumidified yet substantially free of liquid water in an upper region ofthe column. This process is described in International Publication No.WO 02/072510 A1, which is incorporated herein by reference.Alternatively, the monomer can be humidified by employing a solvent thatit humidified. In other words, monomer is mixed with the solvent orcarried by the solvent, and the moisture within the solvent serves tohumidify the monomer stream feed. As those skilled in the art willappreciate, the solvent can be humidified by employing severaltechniques.

The catalyst system employed in practicing the process of this inventionpreferably includes a coordination catalyst system. One type ofcoordination catalyst system includes lanthanide-based systems andanother type includes cobalt-based systems.

In one or more embodiments, a lanthanide system is formed by combining(a) a lanthanide compound, (b) an alkylating agent, and (c) ahalogen-containing compound. Other reagents such as other organometalliccompounds or Lewis bases may also optionally be included. In oneembodiments, the lanthanide catalyst system includes (a) neodymiumneodecanoate, (b) tri isobutyl aluminum, and (c) di isobutyl aluminumchloride or isobutyl aluminum dichloride. Lanthanide catalyst systemsare well known in the art as described in U.S. Pat. Nos. 3,297,667,3,541,063, 3,794,604, 4,461,883, 4,444,903, 4,525,594, 4,699,960,5,017,539, 5,428,119, 5,064,910, and 5,844,050, which are incorporatedherein by reference.

Various lanthanide compounds or mixtures thereof can be employed asingredient (a) of the lanthanide catalyst composition. In one or moreembodiments, these compounds are soluble in hydrocarbon solvents such asaromatic hydrocarbons, aliphatic hydrocarbons, or cycloaliphatichydrocarbons. Hydrocarbon-insoluble lanthanide compounds, however, canbe suspended in the polymerization medium to form the catalyticallyactive species and are also useful.

Lanthanide compounds include at least one atom of lanthanum, neodymium,cerium, praseodymium, promethium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, anddidymium. In particular embodiments, these compounds include neodymium,lanthanum, samarium, or didymium. Didymium is a commercial mixture ofrare-earth elements obtained from monazite sand.

The lanthanide atom in the lanthanide compounds can be in variousoxidation states including but not limited to the 0, +2, +3, and +4oxidation states. Trivalent lanthanide compounds, where the lanthanideatom is in the +3 oxidation state, are particularly useful in one ormore embodiments. Suitable lanthanide compounds include, but are notlimited to, lanthanide carboxylates, lanthanide organophosphates,lanthanide organophosphonates, lanthanide organophosphinates, lanthanidecarbamates, lanthanide dithiocarbamates, lanthanide xanthates,lanthanide β-diketonates, lanthanide alkoxides or aryloxides, lanthanidehalides, lanthanide pseudo-halides, lanthanide oxyhalides, andorganolanthanide compounds.

Various alkylating agents, or mixtures thereof, can be used as component(b) of the lanthanide catalyst composition. Alkylating agents, which mayalso be referred to as hydrocarbylating agents, are organometalliccompounds that can transfer hydrocarbyl groups to another metal.Typically, these agents are organometallic compounds of electropositivemetals such as Groups 1, 2, and 3 metals (Groups IA, IIA, and IIIAmetals). Preferred alkylating agents include organoaluminum andorganomagnesium compounds. Where the alkylating agent includes a labilehalogen atom, the alkylating agent may also serve as thehalogen-containing compound. In one or more embodiments, mixedalkylating systems may be used such as those disclosed in U.S. Ser. No.10/737,591, which is incorporated herein by reference.

The term “organoaluminum compound” refers to any aluminum compoundcontaining at least one aluminum-carbon bond. Organoaluminum compoundsthat are soluble in a hydrocarbon solvent are preferred. Where thealkylating agent is an organoaluminum compound that includes a labilehalogen atom, the organoaluminum compound can serve as both thealkylating agent and the halogen-containing compound.

One class of organoaluminum compounds that can be utilized isrepresented by the general formula AlR_(n)X_(3-n), where each R, whichmay be the same or different, is a mono-valent organic group that isattached to the aluminum atom via a carbon atom, where each X, which maybe the same or different, is a hydrogen atom, a halogen atom, acarboxylate group, an alkoxide group, or an aryloxide group, and where nis an integer of 1 to 3. Each R may be a hydrocarbyl group such as, butnot limited to, alkyl, cycloalkyl, substituted cycloalkyl, alkenyl,cycloalkenyl, substituted cycloalkenyl, aryl, substituted aryl, aralkyl,alkaryl, allyl, and alkynyl groups, with each group preferablycontaining from 1 carbon atom, or the appropriate minimum number ofcarbon atoms to form the group, up to about 20 carbon atoms. Thesehydrocarbyl groups may contain heteroatoms such as, but not limited to,nitrogen, oxygen, boron, silicon, sulfur, and phosphorus atoms.

Another class of suitable organoaluminum compounds is aluminoxanes.Aluminoxanes comprise oligomeric linear aluminoxanes that can berepresented by the general formula:

and oligomeric cyclic aluminoxanes that can be represented by thegeneral formula:

where x is an integer of 1 to about 100, preferably about 10 to about50; y is an integer of 2 to about 100, preferably about 3 to about 20;and where each R¹, which may be the same or different, is a mono-valentorganic group that is attached to the aluminum atom via a carbon atom.Each R¹ may be a hydrocarbyl group such as, but not limited to, alkyl,cycloalkyl, substituted cycloalkyl, alkenyl, cycloalkenyl, substitutedcycloalkenyl, aryl, substituted aryl, aralkyl, alkaryl, allyl, andalkynyl groups, with each group preferably containing from 1 carbonatom, or the appropriate minimum number of carbon atoms to form thegroup, up to about 20 carbon atoms. These hydrocarbyl groups may containheteroatoms such as, but not limited to, nitrogen, oxygen, boron,silicon, sulfur, and phosphorus atoms. It should be noted that thenumber of moles of the aluminoxane as used in this application refers tothe number of moles of the aluminum atoms rather than the number ofmoles of the oligomeric aluminoxane molecules. This convention iscommonly employed in the art of catalysis utilizing aluminoxanes.

Aluminoxanes can be prepared by reacting trihydrocarbylaluminumcompounds with water. This reaction can be performed according to knownmethods, such as (1) a method in which the trihydrocarbylaluminumcompound is dissolved in an organic solvent and then contacted withwater, (2) a method in which the trihydrocarbylaluminum compound isreacted with water of crystallization contained in, for example, metalsalts, or water adsorbed in inorganic or organic compounds, and (3) amethod in which the trihydrocarbylaluminum compound is reacted withwater in the presence of the monomer or monomer solution that is to bepolymerized.

The term organomagnesium compound refers to any magnesium compound thatcontains at least one magnesium-carbon bond. Organomagnesium compoundsthat are soluble in a hydrocarbon solvent are preferred. One class oforganomagnesium compounds that can be utilized is represented by thegeneral formula MgR² ₂, where each R², which may be the same ordifferent, is a mono-valent organic group, with the proviso that thegroup is attached to the magnesium atom via a carbon atom. Each R² maybe a hydrocarbyl group such as, but not limited to, alkyl, cycloalkyl,substituted cycloalkyl, alkenyl, cycloalkenyl, substituted cycloalkenyl,aryl, allyl, substituted aryl, aralkyl, alkaryl, and alkynyl groups,with each group preferably containing from 1 carbon atom, or theappropriate minimum number of carbon atoms to form the group, up toabout 20 carbon atoms. These hydrocarbyl groups may contain heteroatomssuch as, but not limited to, nitrogen, oxygen, silicon, sulfur, andphosphorus atom.

Another class of organomagnesium compounds that can be utilized asingredient (b) is represented by the general formula R³MgX, where R³ isa mono-valent organic group, with the proviso that the group is attachedto the magnesium atom via a carbon atom, and X is a hydrogen atom, ahalogen atom, a carboxylate group, an alkoxide group, or an aryloxidegroup. Where the alkylating agent is an organomagnesium compound thatincludes a labile halogen atom, the organomagnesium compound can serveas both the alkylating agent and the halogen-containing compound.Preferably, R³ is a hydrocarbyl group such as, but not limited to,alkyl, cycloalkyl, substituted cycloalkyl, alkenyl, cycloalkenyl,substituted cycloalkenyl, aryl, allyl, substituted aryl, aralkyl,alkaryl, and alkynyl groups, with each group preferably containing from1 carbon atom, or the appropriate minimum number of carbon atoms to formthe group, up to about 20 carbon atoms. These hydrocarbyl groups maycontain heteroatoms such as, but not limited to, nitrogen, oxygen,boron, silicon, sulfur, and phosphorus atoms. Preferably, X is acarboxylate group, an alkoxide group, or an aryloxide group, with eachgroup preferably containing 1 to 20 carbon atoms.

Various compounds, or mixtures thereof, that contain one or more labilehalogen atoms can be employed as ingredient (c) of the lanthanidecatalyst composition. These compounds may simply be referred to ashalogen-containing compounds. Examples of halogen atoms include, but arenot limited to, fluorine, chlorine, bromine, and iodine. A combinationof two or more halogen atoms can also be utilized. Halogen-containingcompounds that are soluble in a hydrocarbon solvent are preferred.Hydrocarbon-insoluble halogen-containing compounds, however, can besuspended in the oligomerization medium to form the catalytically activespecies, and are therefore useful.

Useful types of halogen-containing compounds include, but are notlimited to, elemental halogens, mixed halogens, hydrogen halides,organic halides, inorganic halides, metallic halides, organometallichalides, and mixtures thereof.

The lanthanide catalyst composition has very high catalytic activity forpolymerizing conjugated dienes into stereoregular polydienes over a widerange of catalyst concentrations and catalyst ingredient ratios. It isbelieved that the catalyst ingredients (a), (b), and (c) may interact toform an active catalyst species. Accordingly, the optimum concentrationfor any one catalyst ingredient is dependent upon the concentrations ofthe other catalyst ingredients. In one embodiment, the molar ratio ofthe alkylating agent to the lanthanide compound (alkylating agent/Ln)can be varied from about 1:1 to about 200:1, in other embodiments fromabout 2:1 to about 100:1, and in other embodiments from about 5:1 toabout 50:1. The molar ratio of the halogen-containing compound to thelanthanide compound (halogen atom/Ln) can be varied from about 0.5:1 toabout 20:1, in other embodiments from about 1:1 to about 10:1, and inother embodiments from about 2:1 to about 6:1. The term molar ratio, asused herein, refers to the equivalent ratio of relevant components ofthe ingredients, e.g., equivalents of halogen atoms on thehalogen-containing compound to lanthanide atoms on the lanthanidecompound.

The lanthanide catalyst composition can be formed by combining or mixingthe catalyst ingredients (a), (b), and (c). Although an active catalystspecies is believed to result from this combination, the degree ofinteraction or reaction between the various ingredients or components isnot known with any great degree of certainty. Therefore, the term“catalyst composition” has been employed to encompass a simple mixtureof the ingredients, a complex of the various ingredients that is causedby physical or chemical forces of attraction, a chemical reactionproduct of the ingredients, or a combination of the foregoing.

The production of polymer by using the lanthanide catalyst systemgenerally employs a catalytically effective amount of the foregoingcatalyst composition. The total catalyst concentration to be employed inthe polymerization mass depends on the interplay of various factors suchas the purity of the ingredients, the polymerization temperature, thepolymerization rate and conversion desired, the molecular weightdesired, and many other factors. Accordingly, a specific total catalystconcentration cannot be definitively set forth except to say thatcatalytically effective amounts of the respective catalyst ingredientsshould be used. In one or more embodiments, the amount of the lanthanidecompound used can be varied from about 0.01 to about 2 mmol, in otherembodiments from about 0.02 to about 1 mmol, and in other embodimentsfrom about 0.05 to about 0.5 mmol per 100 g of conjugated diene monomer.

Catalyst ingredients of the lanthanide system can be charged to thevessel employed in the first stage of this process by using a variety oftechniques and order of addition. In one embodiment, a small quantity ofan organic solvent may be employed as a carrier to either dissolve orsuspend the catalyst ingredients in order to facilitate the delivery ofthe catalyst ingredients to the polymerization system. In yet anotherembodiment, conjugated diene monomer can be used as the catalystcarrier.

In one embodiment, the lanthanide-based system may be pre-formed andaged. Specifically, the catalyst ingredients (e.g., the lanthanidecompound and the alkylating agent, and the halogen-containing compound)can be combined and aged for a period of about one minute to about onehour, in other embodiments from about 3 minutes to about 30 minutes, andin other embodiments from about minutes to about 20 minutes. This agingmay take place in the presence of at least 80% of the monomer to bepolymerized, in other embodiments at least 90% of the monomer to bepolymerized, and in other embodiments at least 99% of the monomer to bepolymerized. The aging process may take place at ambient temperature andpressure. In one or more embodiments, the aging process takes placewithout refrigeration.

The cobalt system can be formed by combining (a) a cobalt compound, (b)an alkylating agent, and (c) a halogen-containing compound. Otherreagents such as other organometallic compounds or Lewis bases may alsooptionally be included. In one embodiment, the cobalt catalyst systemincludes (a) cobalt (II) 2-ethyl hexanoate (b) tri isobutyl aluminum,and (c) di isobutyl aluminum chloride or isobutyl aluminum dichloride.

Various cobalt compounds or mixtures thereof can be employed asingredient (a) of the cobalt catalyst composition. These compounds maybe soluble in hydrocarbon solvents such as aromatic hydrocarbons,aliphatic hydrocarbons, or cycloaliphatic hydrocarbons.Hydrocarbon-insoluble cobalt compounds, however, can be suspended in thepolymerization medium to form the catalytically active species and arealso useful.

The cobalt atom in the cobalt compounds can be in various oxidationstates including but not limited to the two oxidation state. Suitablecobalt compounds include, but are not limited to, cobalt carboxylates,cobalt organophosphates, cobalt organophosphonates, cobaltorganophosphinates, cobalt carbamates, cobalt dithiocarbamates, cobaltxanthates, cobalt β-diketonates, cobalt alkoxides or aryloxides, cobalthalides, cobalt pseudo-halides, cobalt oxyhalides, and organocobaltcompounds.

Various alkylating agents, or mixtures thereof, can be used as component(b) of the cobalt catalyst composition. The alkylating agents definedabove with respect to the lanthanide catalyst system may be used, andtherefore the foregoing discussion is incorporated herein.

Various compounds, or mixtures thereof, that contain one or more labilehalogen atoms can be employed as ingredient (c) of the cobalt catalystcomposition. The alkylating agents defined above with respect to thelanthanide catalyst system may be used, and therefore the foregoingdiscussion is incorporated herein.

The cobalt catalyst composition has very high catalytic activity forpolymerizing conjugated dienes over a wide range of catalystconcentrations and catalyst ingredient ratios. It is believed that thecatalyst ingredients (a), (b), and (c) may interact to form an activecatalyst species. Accordingly, the optimum concentration for any onecatalyst ingredient is dependent upon the concentrations of the othercatalyst ingredients. In one embodiment, the molar ratio of thealkylating agent to the cobalt compound (alkylating agent/Co) can bevaried from about 80:1 to about 15:1, in other embodiments from about60:1 to about 35:1, and in other embodiments from about 45:1 to about55:1. And, the molar ratio of the halogen-containing compound to thecobalt compound (halogen atom/Co) can be varied from about 900:1 toabout 400:1, in other embodiments from about 650:1 to about 500:1, andin other embodiments from about 550:1 to about 600:1. The term molarratio, as used herein, refers to the equivalent ratio of relevantcomponents of the ingredients, e.g., equivalents of halogen atoms on thehalogen-containing compound to cobalt atoms on the cobalt compound.

The catalyst composition may be formed by combining or mixing thecatalyst ingredients (a), (b), and (c). Although an active catalystspecies is believed to result from this combination, the degree ofinteraction or reaction between the various ingredients or components isnot known with any great degree of certainty. Therefore, the term“catalyst composition” has been employed to encompass a simple mixtureof the ingredients, a complex of the various ingredients that is causedby physical or chemical forces of attraction, a chemical reactionproduct of the ingredients, or a combination of the foregoing.

The cobalt catalyst composition is preferably prepared in situ (i.e.within the first reactor) or immediately prior to adding the ingredientsto the reactor such as may occur by combining the ingredients within afeed line. Also, the cobalt catalyst system may include or can bediluted in an inert organic solvent.

The production of polymer by using the cobalt catalyst system generallyemploys a catalytically effective amount of the foregoing catalystcomposition. The total catalyst concentration to be employed in thepolymerization mass depends on the interplay of various factors such asthe purity of the ingredients, the polymerization temperature, thepolymerization rate and conversion desired, the molecular weightdesired, and many other factors. Accordingly, a specific total catalystconcentration cannot be definitively set forth except to say thatcatalytically effective amounts of the respective catalyst ingredientsshould be used. Generally, the amount of the cobalt compound used can bevaried from about 0.01 to about 2 mmol, in other embodiments from about0.02 to about 1 mmol, and in other embodiments from about 0.05 to about0.5 mmol per 100 g of conjugated diene monomer.

The polymerization reaction within the first stage can be carried outunder anaerobic conditions at low temperatures and at or below the vaporpressure of the monomer at the polymerization temperature. Where aorganic solvent is present within the polymerization medium, the solventcan impact the vapor pressure at which the process is conducted.

In one or more embodiments, the polymerization temperature can bemaintained below about 65° C., in another embodiment below about 45° C.,in other embodiments below about 40° C., and in other embodiments belowabout 30° C., with one or more embodiments being from about 15° C. toabout 33° C. (optionally 24° C. to about 32° C.). The polymerizationtemperature can be controlled by externally cooling the vessel in whichthe reaction takes place, internally cooling the reaction by removal ofmonomer vapor, or by using a combination of the two methods. In oneembodiment, monomer vapor can be removed from the vessel and condensedfor future polymerization within the process. For example, anauto-refrigeration loop can be employed whereby monomer vapor can beremoved from the vessel, condensed, and re-circulated back into thevessel. In other embodiments, the vessel can be equipped with anevaporation column that can be controlled by water flow and/or watertemperature. Alternatively, the vapor can be removed, condensed, and themonomer condensate can be fed to a storage tank.

As noted above, the liquid polymerization medium may include unreactedmonomer, polymer, catalyst ingredients, residual solvent, and residualcontaminates. An appropriate head space may be maintained within thevessel to achieve a desired cooling effect from the vaporization ofmonomer. This head space, which includes that volume of the vessel thatis not filled with the polymerization medium but which may containmonomer vapor, can be about 35 to about 65, and in other embodimentsfrom about 45 to about 55 percent by volume of the vessel. One advantageof using the head space is to collapse foam, which thereby minimizesfouling issues with the reactor or peripheral equipment. The volumefraction dedicated to reactor head space can be dependent on thestability of the foam formed and the rate of monomer vaporization (rateof bubble formation) necessary to keep the reaction runningisothermally. Some factors that may contribute to foam stability includethe concentration of polymer, polymer molecular weight, and polymermicrostructure.

The polymerization medium, which can be in the liquid phase, may bemixed by employing mixing techniques that are known in the art. Forexample, mixing may be achieved by the use of a pitched blade, Rushton,flat blade turbine, or helical mixer, as well as anchors or anycombination thereof. Notably, certain embodiments of this invention canadvantageously be carried out by employing conventional reactorequipment that may be employed in solution polymerizations. This abilityto advantageously use conventional equipment results, in large part,from the fact that the low conversions maintained in the first stage donot typically result in or produce extra ordinarily high viscosities.

The vessel employed in practicing the first stage of the process of thisinvention can include a variety of reactors. In one embodiment, acontinuously-stirred tank reactor (CSTR) can be employed. A CSTR may becharacterized by a tank with a top mounted agitator running along thevertical axis of the tank. The length to diameter ratio of the reactormay be less than 3 and in other embodiments less than 2.

Once the maximum polymerization or monomer conversion is achieved in thefirst stage of the process, the polymerization medium is removed fromthe first vessel employed in the first stage and transferred to a secondstage, which takes place in a second vessel. Transfer of polymer fromthe first stage to second stage may be accomplished by employing a pump.The pump speed can control the discharge rate out of the first vesseland the input into the second vessel. A liquid level device can beemployed to measure the height within the first vessel and modulate thespeed of the pump that transfers product solution from the first vesselto the second vessel. Also, in one or more embodiments, ingredients orother materials can be added at or near the pump. For example,antioxidant or solvent, or terminating agent can be added at the pump.

Within this second stage, the polymerization reaction can be terminated.Alternatively, the polymerization reaction can be terminated between thefirst and second stages. In other words, the polymerization reaction canbe terminated by adding an appropriate terminating agent to a feed lineor pump between the first vessel and second vessel. In other words,within this latter embodiment, the termination of the polymerizationwill begin prior to the polymerization medium entering the secondvessel.

An optional aspect of the second stage of the process includes theseparation of solvent and unreacted monomer from the polymer product. Inthose embodiments where insufficient or less than a desired amount ofsolvent or unreacted monomer may not be removed from the polymer productin the second stage, additional treatment of the polymer product canoccur.

The polymerization reaction may be quenched prior to significant monomerconversion (or polymerization) within the second stage. Thepolymerization reaction may be terminated prior to a total monomerconversion of less than about 25%, where the total monomer conversionrefers to the amount or degree of monomer conversion in both the firstand second stages. In other embodiments, the polymerization reaction canbe terminated prior to a total monomer conversion of less than 22%, inother embodiments less than 20%, and in other embodiments less than 17%.For example, if the polymerization medium is removed from the vessel inthe first stage at a point where the monomer conversion is about 15%,then the polymerization reaction can be terminated within the secondstage prior to the conversion of another 10% of the monomer, which wouldyield a total monomer conversion of about less than 25%. Those skilledin the art will be able to determine the extent to which thepolymerization may be allowed to continue within the second stagewithout undue calculation or experimentation. Stated another way, thedegree of polymerization within the second stage may be limited suchthat less than 5%, in other embodiments less than 3%, and in otherembodiments less than 1% of the total monomer added to the system ispolymerized within the second stage.

The polymerization reaction can be terminated by using many of thetechniques known in the art. For example, useful techniques include theaddition of a protonating or quenching agent, the addition of a couplingagent, the addition of a functionalized terminator, or a combinationthereof, which react or interact with living polymer chains and preventfurther growth or polymerization. In one or more embodiments, sufficientterminating agent can be added to prevent the aluminum-alkyl complexesfrom having an appreciable impact on the polymer product.

For example, reactive or reacting polymers can be quenched or protonatedby reacting them with a proton source. Compounds or agents that can beemployed to provide a proton source include water, alcohols (e.g.,isopropyl alcohol), butylated hydroxy toluene (BHT), tert-butyl-catechol(TBC), as well as various other glycols and organic acids.

Also, the living or pseudo-living polymers can be reacted with acompound that will not only terminate the polymerization but alsoend-functionalize or couple the polymer. In one or more embodiments, theability to react the polymer with a functionalizing agent may resultfrom the living character or pseudo-living character of the polymer.Exemplary functionalizing or coupling agents include, but are notlimited to, metal halides, metalloid halides, alkoxysilanes,imine-containing compounds, esters, ester-carboxylate metal complexes,alkyl ester carboxylate metal complexes, aldehydes or ketones, amides,isocyanates, isothiocyanates, imines, and epoxides. These types ofcoupling and functionalizing agents are described in, among otherplaces, U.S. Pat. Nos. 4,906,706, 4,990,573, 5,064,910, 5,567,784,5,844,050, 6,977,281, and 6,992,147; Japanese Patent Application Nos.05-051406A, 05-059103A, 10-306113A, and 11-035633A, which areincorporated herein by reference. The polymer, which may be living orpseudo-living, may be contacted with a coupling or functionalizing agentprior to contacting the polymerization mixture with the terminator or anantioxidant.

In one or more embodiments, the amount of coupling or functionalizingagent employed may vary from about 0.01 to about 100 moles, in otherembodiments from about 0.1 to about 50 moles, and in other embodimentsfrom about 0.2 to about 25 moles per mole of the living or pseudo-livingpolymer.

In certain embodiments, at least 80%, in other embodiments at least 90%,in other embodiments at least 95%, and in other embodiments at least 99%of the monomer (i.e., unreacted monomer) within the polymerizationmedium can be removed in the second stage.

In certain embodiments, at least 50%, in other embodiments at least 80%,in other embodiments at least 95%, and in other embodiments at least 99%of any solvent present within the polymerization medium can be removedduring the second stage.

Monomer and solvent can be removed by employing a variety of techniques,or a combination thereof, as is known in the art. For example, thetemperature of the polymerization medium can be increased or maintainedat a temperature sufficient to volatize the monomer. Also, the pressurewithin the vessel in which the second stage of the process is conductedcan be decreased, which may likewise assist in the volatilization ofmonomer. Still further, the polymerization medium can be agitated, whichmay further assist in the removal of monomer from the polymerizationmedium. In one embodiment, a combination of heat, decreased pressure,and agitation can be employed.

In one embodiment, the temperature of the polymerization medium withinthe second stage can be maintained at a temperature in excess of about60° C., in other embodiments in excess of about 66° C., and in otherembodiments in excess of about 71° C.

In one embodiment, the pressure within the vessel in which the secondstage of the process is conducted can be maintained below about 10 kPa,in other embodiments below about 105 kPa, and in other embodiments belowabout 100 kPa.

Various methods, which are known by those skilled in the art, can beemployed to agitate the polymerization medium within the second stage ofthe process. Agitation can expose greater surface area of thepolymerization medium and thereby facilitate the evolution of unreactedmonomer.

In one embodiment, a devolatizer can be employed as the vessel in whichthe second stage of the process is conducted. Devolatizers can include adevolatizing extruder, which typically includes a screw apparatus thatcan be heated by an external heating jacket. These extruders are knownin the art such as single and twin screw extruders.

Alternatively, devolatizers can include extruder-like apparatus thatinclude a shaft having paddles attached thereto. These extruder-likeapparatus can include a single shaft or multiple shafts. The shaft canbe axial to the length of the apparatus and the flow of polymer orpolymerization medium. The polymer or polymerization medium may beforced through the apparatus by using a pump, and the shaft rotatesthereby allowing the paddles to agitate the polymer or polymerizationmedium and thereby assist in the evolution of unreacted monomer. Thepaddles can be angled so as to assist movement of the polymerizationmedium through the devolatilizer, although movement of thepolymerization medium through the devolatilizer can be facilitated bythe pump that can direct the polymerization medium into thedevolatilizer and may optionally be further assisted by an extruder thatmay optionally be attached in series or at the end of the devolatilizer(i.e., the extruder helps pull the polymerization medium through thedevolatilizer).

Devolatilizers can further include backmixing vessels. In general, thesebackmixing vessels include a single shaft that includes a blade that canbe employed to vigorously mix and masticate the polymerization medium.

In certain embodiments, combinations of the various devolatilizingequipment can be employed to achieve desired results. These combinationscan also include the use of extruders. In one example, a single shaft“extruder-like” devolatilizer (e.g., one including paddles) can beemployed in conjunction with a twin screw extruder. In this example, thepolymerization medium first enters the “extruder-like” devolatilizerfollowed by the twin screw extruder. The twin screw extruderadvantageously assists in pulling the polymerization medium through thedevolatilizer. The paddles of the devolatilizer can be adjusted to meetconveyance needs.

In another example, a twin shaft “extruder-like” devolatilizer can beemployed. In certain embodiments, the paddles on each shaft may bealigned so as to mesh with one another as they rotate. The rotation ofthe shafts can occur in the same direction or in opposite directions.

In yet another example, a backmixing volatilizing vessel can be followedby a twin screw extruder, which can then be followed by a twin shaftextruder-like devolatilizing vessel, which can then be following by atwin screw extruder.

Devolatilizing equipment is known in the art and commercially available.For example, devolatilizing equipment can be obtained from LIST(Switzerland); Coperion Werner & Phleiderer; or NFM Welding Engineers,Inc. (Ohio). Exemplary equipment available from LIST include DISCOTHERM™B, which is a single shaft “extruder-like” devolatilizer includingvarious mixing/kneading bars or paddles; CRP™, which is a dual shaft“extruder-like” devolatilizer wherein each shaft correlates with theother; ORP™, which is a dual shaft devolatilizer wherein each shaftrotates in an opposite direction to the other.

In one or more embodiments, the devolatilizers are attached to a monomerrecovery system. In other words, as monomer is separated from thepolymer product, the monomer can be directed to a cooling or evaporationsystem. The monomer that is recovered can optionally be returned as araw material to the first stage.

In one or more embodiments, an antioxidant can be added to thepolymerization medium in the second stage. Antioxidant can be addedprior to entry into the second vessel (e.g., devolatizer), while in thevessel employed in the second stage, or thereafter. Useful antioxidantsinclude those available under the tradenames Irganox™ 1076 and Irganox™1520. Other ingredients that are conventionally employed in rubberproduction can also be added during the second stage.

The second stage of the process of this invention may include or befollowed by further processing of the polymerization medium. In oneembodiment, further processing of the polymerization medium may includefurther desolventization or removal of monomer from the polymerizationmedium. This can be achieved by employing a variety of techniques whichare known in the art. In one embodiment, further desolventization andmonomer removal can be achieved by processing the polymerization mediumthrough yet another extruder such as a single or twin screw extruder.The polymer product can then be baled, and in certain embodiments dicedor pelletized prior to baling.

An exemplary system configuration for carrying out the process can bedescribed with reference to FIG. 1, which shows configuration 100. Ingeneral, the first stage of the process 110 can be achieved using acontinuously stirred tank reactor 111. This reactor includes mixingapparatus 115, which includes mixing blades 112 attached to shaft 113,which is driven by motor 114. Monomer and catalyst ingredients, as wellas other raw materials that may be employed, can be added to reactor 111via inlet 118 (although other inlets may exist).

The reactor 111 may also be equipped with an auto-refrigeration loop120. Vaporized monomer can enter loop 120 via outlet 121 and directedtoward condenser 122 by employing the appropriate conduit. The condensedmonomer (i.e., liquid monomer) can be introduced back into reactor 111via inlet 125. Alternatively, the condensed monomer can be directedtoward and stored within a storage tank (not shown).

The polymerization medium, which may include polymer and monomer, can becontinuously removed from reactor 111 via outlet 126. The polymerizationmedium can then be transferred to the second stage 130 by using pump127.

Within second stage 130, the polymerization medium can be pumped fromfirst stage 110 through appropriate conduit to devolatilizer 131, whichmay include various paddles 132 attached to shaft 133 that can be drivenby a hydraulic or electric motor. The polymerization medium entersdevolatilizer 131 at inlet 135. The force applied to the polymerizationmedium by pump 127 forces the polymerization medium throughdevolatilizer 131.

Monomer vapor that may evolve from the polymerization medium withindevolatilizer 131 can exit devolatilizer 131 via outlet 136. Thismonomer vapor can then be compressed by compressor 137, transferred tocondenser 138 via appropriate conduit, and the condensed monomer (i.e.,liquid monomer) can be transferred back to first stage 110 usingappropriate conduit and added to reactor 111 at inlet 129.Alternatively, the condensed monomer can then be stored within a storagetank (not shown).

The flow of polymerization medium into devolatilizer 131 via inlet 135together with the flow of polymerization medium out of devolatilizer 131via outlet 140 can be controlled so as to not completely fill the volumeof the devolatilizer 131 and thereby create headspace 141.

Polymerization medium exiting devolatilizer 131 via outlet 140 mayinclude less unreacted monomer than the polymerization medium enteringdevolatilizer 131 at inlet 135. This reduction in monomer may be due tothe fact that unreacted monomer has been removed via outlet 136 andcondensed. Nonetheless, the polymerization medium exiting devolatilizer131 at outlet 140 may contain residual unreacted monomer, and thereforepolymerization medium may be transferred by using appropriate conduit,to twin screw extruder 145. The polymerization medium enters extruder145 at inlet 146 and exits extruder 145 via outlet 147. Extruder 145includes twin screw 148, which may be driven by hydraulic or electricmotor. Polymer product exiting extruder 145 at outlet 147 can then befurther treated by employing techniques that are known in the artincluding, but not limiting to, bailing.

The introduction of raw materials into reactor 111 may vary based uponthe type of catalyst system employed. In one embodiment, where a cobaltcatalyst system is employed, the introduction of catalyst and rawmaterial can be achieved by employing a system as set forth in FIG. 2.Namely, the system 150 for introducing catalyst and raw material intoreactor 111 may include a first feed line 155 for the introduction ofalkylating agent (e.g., tributyl aluminum) and a halogen source (e.g.,dibutyl aluminum chloride) via inlet 156. The alkylating agent can befed to first feed line 155 via alkylating agent line 153, and halogensource can be fed to first feed line 155 via halogen source feed line154. Accordingly, in this embodiment, the alkylating agent and halogensource are combined within first feed line 155 prior to introductioninto reactor 111 via inlet 156. The cobalt compound can be injected intoreactor 111 at inlet 158 via a second feed line 157. Monomer may enterinto reactor 111 via third feed line 159 at inlet 160 (i.e., it can beadded along with the alkylating agent and halogen source).Alternatively, monomer can be added via third feed line 159 into aseparate or different inlet (not shown). Other ingredients, such assolvent and a gel inhibitor (e.g., 1,2-butadiene) can be injected intoreactor 111 at various other inlets by employing similar type feedlines.

In another embodiment, where a lanthanide-based catalyst system isemployed, an exemplary system for introducing catalyst ingredient andraw material into reactor 111 is shown in FIG. 3. This system 170includes catalyst aging vessel 175. Lanthanide compound (e.g., neodymiumversatate), alkylating agent (e.g., tributyl aluminum), and monomer canbe injected into aging vessel 175 at inlet 176 via first feed line 171.The lanthanide compound can be fed to first feed line 171 via lanthanidefeed line 172, alkylating agent can be fed to first feed line 171 viaalkylating agent feed line 173, and monomer can be fed to first feedline 171 via monomer feed line 174. Monomer may enter first feed line171 prior to the catalyst ingredients. Accordingly, in this embodiment,the lanthanide compound, alkylating agent, and monomer may be combinedwithin first feed line 171 prior to introduction into aging vessel 175at inlet 176.

Aging vessel 175 can be designed to allow the lanthanide compound,alkylating agent, and monomer to age for multiple time intervals priorto introduction to reactor 111 at inlet 190 via fourth feed line 191.For example, aging vessel 175 can have multiple outlets (e.g., firstoutlet 181, second outlet 183, third outlet 185, and fourth outlet 187)whereby each outlet releases a mixture of lanthanide compound,alkylating agent, and monomer, that has been aged for distinct timeintervals. For example, mixture exiting outlet 181 can be aged for about0 to 1 minutes, mixture exiting outlet 183 can be aged for about 5minutes, mixture exiting outlet 185 can be aged for about 10 minutes,and the mixture exiting outlet 187 can be aged for about 20 minutes.These various mixtures can then be introduced to fourth feed line 191via exit lines 180, 182, 184, and 186 respectively. In one or moreembodiments, aging vessel 175 can be operated with only one outlet openso that the catalyst delivered to the process may have a substantiallyconstant aging time rather than a distribution of aging times. Thesource of halogen can be fed to reactor 111 via halogen feed line 192 atinlet 193.

In one or more embodiments, the process of this invention may allow forthe production of polymers having targeted properties. In certainembodiments, the process can advantageously be employed to synthesizepolybutadiene having particular characteristics that allow thepolybutadiene to be employed for specialized uses.

In one or more embodiments, where a lanthanide-based catalyst system isemployed, the process of this invention may produce polybutadiene havinga molecular weight distribution of less than 4, in other embodimentsless than 3.5, in other embodiments less than 3, and in otherembodiments less than 2.5.

In one or more embodiments, where a lanthanide-based catalyst system isemployed, the process of this invention may advantageously be employedto produce polybutadiene having a cis content in excess of about 97, inother embodiments in excess of about 98, and in other embodiments inexcess of about 99.

In one or more embodiments, where the lanthanide-based catalyst systemis employed, the process of this invention may advantageously allow forthe functionalization of a significant number of polybutadiene ends. Asthose skilled in the art will appreciate, the ability to functionalizethe ends of the polybutadiene may be related to the reactiveness of thepolymer with a nucleophile (i.e., the ability of the polymer to undergonucleophilic attack). This is commonly referred to as the living orpseudo-living nature of the polymer prior to terminating or quenchingthe polymerization, which termination or quenching greatly reduces orprecludes this reactivity to a nucleophile. In one or more embodiments,the process of this invention may advantageously provide apolymerization medium to the second stage of the process wherein thepolymerization medium at this point is characterized by having at least10% of the polymers with reactive or live ends, in other embodiments atleast 20% of the polymers with reactive or live ends, in otherembodiments greater than 40% of the polymers with reactive or live ends,and in other embodiments greater than 60% of the polymers with reactiveor live ends.

Also where a cobalt-based catalyst system is employed, the process ofthis invention may advantageously produce highly branched polymers.These highly branched polymers may be characterized by having a solutionviscosity less than that of a linear polymer of the same molecularweight.

In one or more embodiments, the process of the present invention can beemployed to achieve certain target Mooney viscosities for the polymerproduct. For example, where a neodymium-based catalyst system isemployed, a Mooney viscosity (ML₁₊₄@100° C.) of from about 15 to about45, in other embodiments from about 20 to about 40, and in otherembodiments from about 25 to about 35 can be achieved. Alternatively,where a cobalt-based catalyst system is employed, a Mooney viscosity offrom about 30 to about 55, in other embodiments from about 33 to about48, and in other embodiments from about 38 to about 45 can be achieved.

Various modifications and alterations that do not depart from the scopeand spirit of this invention will become apparent to those skilled inthe art. This invention is not to be duly limited to the illustrativeembodiments set forth herein.

1. A method for continuously polymerizing conjugated diene monomer, themethod comprising: continuously introducing conjugated diene monomer, acobalt-based catalyst system, and optionally organic solvent to a firstvessel to form a mixture, where the monomer polymerizes to form polymerin the presence of the cobalt-based catalyst system, and where themixture includes less than 50% by weight organic solvent based on thetotal weight of the monomer, polymer, and solvent; and continuouslyremoving the mixture from the first vessel prior to maximum monomerconversion of up to about 20%, where the method produces branchedpolydienes characterized by having a solution viscosity less than thatof a linear polymer of the same molecular weight.
 2. The method of claim1, further comprising maintaining the mixture at a temperature belowabout 65° C.
 3. The method of claim 1, further comprising the step ofmaintaining the mixture at a temperature below about 30° C.
 4. Themethod of claim 1, where the mixture includes less than 20% by weightorganic solvent.
 5. The method of claim 4, where said step ofcontinuously removing occurs prior to a maximum conversion of up to 15%.6. The method of claim 5, where said step of continuously removingoccurs prior to a maximum conversion of up to 12%.
 7. A method forcontinuously polymerizing conjugated diene monomer, the methodcomprising: continuously introducing conjugated diene monomer, alanthanide-based catalyst system, and optionally organic solvent to afirst vessel to form a mixture, where the monomer polymerizes to form areactive polymer in the presence of the lanthanide-based catalystsystem, and where the mixture includes less than 20% by weight organicsolvent based on the total weight of the monomer, polymer, and solvent;and continuously removing the mixture from the first vessel prior to amaximum monomer conversion of up to about 20%; transferring the mixtureremoved from the first vessel to a second vessel; and continuouslyquenching the mixture within the second vessel.
 8. The method of claim7, wherein a head space of about 30 to about 70% of the volume of thesecond vessel is maintained within the second vessel.
 9. The method ofclaim 7, further comprising maintaining the mixture at a temperaturebelow about 65° C. within the first vessel.
 10. The method of claim 7,further comprising the step of maintaining the mixture at a temperaturebelow about 30° C. within the first vessel.
 11. The method of claim 7,where the mixture includes less than 10% by weight organic solvent. 12.The method of claim 11, where the mixture includes less than 5% byweight organic solvent.
 13. The method of claim 12, where the mixtureincludes less than 2% by weight organic solvent.
 14. The method of claim7, further comprising the step of treating the polymerization mixturewith functionalizing agent after said step of continuously removing.